1. Field of the Invention
The present invention relates to a process for producing a 4,4'-dihydroxybiphenyl having high purity by a special purification system after hydrolyzing 4,4'-dibromobiphenyl.
2. Description of the Prior Arts
4,4'-Dihydroxybiphenyl is useful as an antioxidant of a resin; an intermediate of a dye; and a starting material for polyesters, polyepoxides, polyurethanes and polycarbonates. This compound has been considered as a starting material for heat resistant resins. Accordingly, it has been required to supply 4,4'-dihydroxybiphenyl having a purity of greater than 98% and a content of inorganic salts of less than 30 ppm. This is relatively severe requirement. Since 4,4'-dihydroxybiphenyl has high melting point and low solubility etc., it is quite difficult to obtain 4,4'-dihydroxybiphenyl having the required quality.
In the conventional process for producing 4,4'-dihydroxybiphenyl, 4,4'-biphenyl disulfonic acid is molten with alkali metal hydroxide and then, hydrazine is added to the alkaline aqueous solution of the reaction product and then, it is neutralized to obtain a colorless 4,4'-dihydroxybiphenyl (Japanese Unexamined Patent Publication No. 68154/1977). However, this conventional process has an industrial difficulty on a wasted water treatment in the sulfonation.
The inventors have studied on a process for producing 4,4'-dihydroxybiphenyl by a hydrolysis of 4,4'-dihalobiphenyl. (Japanese Unexamined Patent Publication No. 22347/1979). In this process, a copper compound is used as a catalyst for hydrolyzing 4,4'-dibromobiphenyl in an alkaline aqueous solution, whereby 5-15% of by-products such as phenylphenols formed by dehalogenation and alkali soluble macromolecular compounds, are caused by the side reaction. When 4,4'-dihydroxybiphenyl is obtained by neutralizing the alkaline reaction mixture, these by-products are included in the product as the impurities. It has been difficult to obtain 4,4'-dihyroxybiphenyl having high purity. Moreover, crystals of 4,4'-dihyroxybiphenyl precipitated by neutralizing the alkaline reaction mixture are fine. The filtration is not easily performed and the inorganic salts such as sodium halides, are not easily removed from the filtered cake of the product obtained by the filtration, even though filtered cake is washed with water. It is impossible to purify 4,4'-dihydroxybiphenyl by a distillation so as to give the quality required for the demands in view of the characteristics of 4,4'-dihydroxybiphenyl.